The present invention is directed to an apparatus and a system for measuring a physical property, such as thickness, of a sheet material. The present invention is more specifically directed toward a caliper gauge for measuring the thickness of a sheet of paper which is in the process of being manufactured by a papermaking machine and, therefore, is moving at a high rate of speed through the caliper gauge.
Various types of caliper gauges are known in sensor technology and are used for measuring the thickness of rapidly moving sheet material. One type of caliper gauge is called a "contacting caliper gauge." Contacting caliper gauges typically have two opposing pads which are forced into contact with opposite sides of the sheet. The distance between the pads is measured and directly related to the sheet thickness or "caliper."
Under some situations, however, contacting caliper gauges may suffer from certain shortcomings. For example, many modern paper mills now manufacture paper from wood pulp obtained from recycled materials, such as scrap magazines and discarded books. Such materials often contain glue, which originally functioned as a binding material to hold one edge of the papers together. It is generally too expensive to remove such glue from the recycled pulp during the papermaking process and, therefore, the glue ends up in the final product. Although the glue may not substantially degrade the quality of the final product, it causes formidable problems in obtaining accurate measurement of the caliper of the paper.
The problem of inaccurate caliper measurement is caused because paper mills manufacture paper at temperatures at which the glue is beginning to solidify. As the rapidly moving sheet passes between the opposing caliper pads, the hot glue rapidly builds up on the contacting surface of the pads forcing the pads away from the surface of the sheet. However, as previously mentioned, contacting caliper gauges determine sheet thickness based upon the measured distance between the opposing pads. Thus, the build up of the hot glue produces an inaccurate caliper measurement by making the sheet appear thicker to the caliper gauge than it actually is. In fact, conventional contacting caliper gauge designs often tend to form significant build up very rapidly. Obviously, this is entirely unacceptable when attempting to measure paper caliper to within 1 micron accuracy under a wide range of conditions, as is necessary in certain paper manufacturing situations.
Caliper designers have given little, if any, consideration to the overall shape of the caliper pads in preventing build up of glue on the sheet contacting surfaces of the pads. For example, conventional caliper pads have been essentially disk-shaped, with smooth, rounded edges to avoid snagging the sheet. However, the smooth rounded trailing edge of the pad provides a surface to which hot glue can initially adhere. This initial hot glue provides a slight obstruction on which more glue builds up. Additional glue then begins to form upstream (relative to the direction of sheet movement) from the obstruction on the sheet contacting surface of the pad. Eventually, the glue build up can occupy most, if not all, of the sheet contacting surface area, forcing the pad off of the sheet, thereby producing inaccurate measurements.